In recent years, as a high-density and large-capacity storage medium, a high-density and large-capacity optical disk medium called DVD (Digital Versatile Disc) has been put to practical use, and is widely prevalent as an information storage medium on which a large amount of information, e.g., moving pictures, can be recorded. On such an information storage medium, information is recorded with marks and spaces.
With reference to FIG. 26, a conventional optical head device will be described. FIG. 26 is a diagram showing a conventional optical head device 300.
The optical head device 300 is mounted in an optical information processing device (not shown) which performs recording/reproduction of information. In this optical information processing device, an optical storage medium 21 is irradiated with three light beams for detecting a tracking error signal (see, for example, Patent Document 1).
With reference to FIG. 26, a light source 1 such as a semiconductor laser element emits a divergent light beam 10 of linearly polarized light having a wavelength λ of 405 nm. The divergent light beam 10 having been emitted from the light source 1 is converted into collimated light by a collimating lens 11 having a focal length f1 of 15 mm, and thereafter enters a diffraction grating 12. The light beam 10 having irradiated the diffraction grating 12 is split into three light beams of 0th and ±1st order diffracted light. The 0th order diffracted light is a main light beam 10a with which to perform recording/reproduction of information. The ±1st order diffracted light is two sub-light beams 10b and 10c to be used for a differential push-pull (hereinafter referred to as DPP) technique for detecting a tracking error (hereinafter referred to as TE) signal.
In the diffraction grating 12, in order to avoid unwanted recording occurring with the sub-light beams, the ratio of diffraction efficiencies between the 0th order diffracted light 10a and either 1st order diffracted light 10b or 10c is usually set to 10:1 to 20:1, and herein is 20:1. The three light beams 10a to 10c having been generated in the diffraction grating 12 are transmitted through a polarization beam splitter 13, transmitted through a ¼ wavelength plate 14 so as to be converted into circularly polarized light, and then converted into convergent light beam by an objective lens 15 having a focal length f2 of 2 mm, transmitted through a transparent substrate 21a of the optical storage medium 21, and converged onto an information recording layer 21c. The optical storage medium 21 includes two information recording layers 21b and 21c. In FIG. 26, the light beam 10 having been converged by the objective lens 15 comes to a focal point at the information recording layer 21c. 
It is assumed that there is a distance d2 of 100 μm from the light incident surface of the optical storage medium 21 to the information recording layer 21c, and that there is a distance d1 of 25 μm between the information recording layer 21b and the information recording layer 21c. The tracks which are formed in the information recording layers 21b and 21c have a period tp (FIG. 27) of 0.32 μm. The opening of the objective lens 15 is restricted through the aperture 16, with a numerical aperture NA of 0.85. The transparent substrate 21a has a thickness of 0.1 mm and a refractive index n of 1.62. The information recording layers 21b and 21c each have an equivalent reflectance of about 4 to 8%. Herein, an equivalent reflectance represents the light amount of a light beam which, after being reflected at the information recording layer 21b or 21c, again goes out from the optical storage medium 21, assuming that the light beam entering the optical storage medium 21 has a light amount of 1. Although the information recording layer 21c absorbs or reflects most of the light amount of the incident light beam, the information recording layer 21b transmits about 50% of the light amount of the incident light beam, while absorbing or reflecting the other 50% light amount, thus allowing the light beam to reach the information recording layer 21c. 
FIG. 27 is a diagram showing relative positioning of light beams and tracks on the information recording layer 21c. A continuous groove to become the tracks is formed on the information recording layers 21b and 21c, and information is to be recorded in the groove. Tracks Tn−1, Tn and Tn+1 have a track pitch tp of 0.32 μm. When the main light beam 10a is converged on the track Tn, the sub-light beam 10b is converged between the track Tn−1 and the track Tn, and the sub-light beam 10c is converged between the track Tn and the track Tn+1. There is an interval L of 0.16 μm between the main light beam 10a and the sub-light beams 10b and 10c along a direction which is orthogonal to the tracks.
The light beams 10a to 10c having been reflected from the information recording layer 21c are transmitted through the objective lens 15 and the ¼ wavelength plate 14 so as to be converted into linearly polarized light which has a 90° difference with respect to the forward path, and thereafter are reflected by the polarization beam splitter 13. The light beams 10a to 10c having been reflected by the polarization beam splitter 13 are transmitted through a converging lens 25 having a focal length f3 of 30 mm so as to be converted into convergent light, and enter a photodetector 30 via a cylindrical lens 26. When being transmitted through the cylindrical lens 26, astigmatism is imparted to the light beams 10a to 10c. 
FIG. 28 is a diagram showing relative positioning of the photodetector 30 and the light beams 10a to 10c. The photodetector 30 includes eight photosensitive portions 30a to 30h, such that: the photosensitive portions 30a to 30d receive the light beam 10a; the photosensitive portions 30e to 30f receive the light beam 10b; and the photosensitive portions 30g to 30h receive the light beam 10c. The photosensitive portions 30a to 30h output electrical signals I30a to I30h (not shown), respectively, which are in accordance with the received light amounts. A focus error (hereinafter referred to as FE) signal is obtained through a calculation according to astigmatism technique, employing the signals I30a to I30d having been output from the photodetector 30. For example, an FE signal is obtained through the calculation of (I30a+I30c)−(I30b+I30d). Moreover, a TE signal is obtained through a calculation according to DPP technique. For example, a TE signal is obtained through the calculation of {(I30a+I30d)−(I30b+I30c)}−C·{(I30e+I30g)−(I30f+I30h)}. Herein, C is a coefficient which is determined by a ratio of diffraction efficiencies, at the diffraction grating 12, between the 0th order diffracted light and either 1st order diffracted light. The FE signal and the TE signal are subjected to desired levels of amplification and phase compensation, and thereafter supplied to actuators 31 and 32 for moving the objective lens 15, whereby focus control and tracking control are performed. Moreover, a reproduction signal (hereinafter referred to as RF signal) representing the information which is recorded in the information recording layer 21c is obtained through the calculation of I30a+I30b+I30c+I30d. 
[Patent Document 1] Japanese Laid-Open Patent Publication No. 3-005927 (pp. 5 to 8, FIG. 2)